Influence of Laser Repetition Rate, Fluence and Spot Size on LA ‐Q‐ ICP ‐ MS / MS ⁸⁷ Sr / ⁸⁶ Sr Precision: Insights from Apatites and Silicate Glasses

Recent advances enable in situ 87 Sr/ 86 Sr analysis of geological materials at micrometre‐scale using LA‐Q‐ICP‐MS/MS, which employs reaction gases to eliminate isobaric interferences. However, single‐detector quadrupole ICP‐MS typically yields lower precision than multi‐collector systems because isotopes are measured sequentially rather than simultaneously. Improving LA‐Q‐ICP‐MS/MS 87 Sr/ 86 Sr precision would enhance its applicability in many geological and environmental scenarios. Here, we assess the effects of laser repetition rate, laser fluence and spot size on measurement precision of Sr isotopes using LA‐Q‐ICP‐MS/MS with N 2 O as reaction gas. Thirteen reference materials (RMs) comprising seven apatites (Durango, Emerald Lake, Madagascar, McClure, Mud Tank, Otter Lake and 401) and six silicate glasses (NIST SRM 610, NIST SRM 612, USGS BCR‐2G, USGS BHVO‐2G, USGS BIR‐1G, USGS GSD‐1G) were analysed by systematically varying laser repetition rate, fluence and spot diameter. Our results show that increasing laser repetition rate improves signal intensity and stability, resulting in enhanced analytical precision. After mass bias correction, 87 Sr/ 86 Sr precision improved from > 4‰ at 10 Hz to < 3‰ at 20 Hz (1 s ). Notably, 20 Hz exceeds the 5–10 Hz commonly used in most LA‐ICP‐MS Sr isotope studies. The positive correlation between repetition rate and precision was observed in both apatite and glass RMs. Although analyses conducted at 5 J cm ‐2 produce higher 88 Sr counts than those at 3 J cm ‐2 , both fluences show similar improvements in 87 Sr/ 86 Sr precision with increasing spot diameter. While both larger spot sizes and higher repetition rates increased signal intensity, the latter yields more stable signals. For LA‐Q‐ICP‐MS/MS measurement of 87 Sr/ 86 Sr in apatite and similar geological materials, repetition rates ≥ 12 Hz are recommended under conditions tested here. Among the glasses, USGS BHVO‐2G yielded the best and most consistent analytical uncertainty, indicating its suitability as a reference material for Sr isotope measurement.